As frequency of operation is increased the structure carrying electronic signals changes. At microwave frequencies it is common to have combined ground and signal structures to minimize loss and maintain signal integrity. To modify signals it is common to use inductor and capacitor effects to enable such changes as filtering and delaying. It is desired in many cases to be able to tune or modify the intensity of the effect, and one way of accomplishing this is to vary the value of the capacitor.
Variable dielectric materials exhibit re-orientable spontaneous polarization, resulting in a variable dielectric constant. The dielectric constant can be varied when an electric field is applied, a key property rendering it appealing for applications in tunable microwave devices. Tunable dielectrics include ferroelectrics such as Barium Strontium Titanate (BaxSr1-xTiO3, where x is between 0 and 1, preferably between 0.1 and 0.9, and more preferably between 0.4 and 0.6, or BST) and other materials such as Bismuth Zinc Niobate (BZN). Studies of variable dielectric microwave devices started in the early 1960s, however it is only recently that the interest is rejuvenated, thanks to the maturity of process in making low-loss, high-tunability ferroelectric materials. BST is among the most studied ferroelectric materials due to its high dielectric constant, high breakdown voltage, and relative low loss. Examples of microwave applications include, but are not limited to, varactors, tunable filters, phase shifters, oscillators, tunable matching networks, resonators, and delay lines.
Variable dielectric materials can be made in the form of bulk, thin films and thick films. Ferroelectric films show moderate temperature-dependent dielectric constant variation (temperature coefficient). Modifications in film deposition have resulted in improved dielectric constant-temperature responses, such as the use of multilayered films, each layer having different Curie temperatures. Optimization in circuit design can also alleviate the temperature dependence of dielectric constant. BZN has an even lower temperature coefficient and lower loss, but the dielectric constant is also lower.
Both distributed and lumped-element microwave devices have been designed employing ferroelectric materials. Ferroelectric materials, such as BST, exhibit high dielectric constant (e.g., 500 for BST thin films and 10,000 for bulk BST), having advantages of reducing device sizes. However, this makes impedance matching difficult in a distributed circuit such as a coplanar waveguide (CPW). Lumped element circuits, on the other hand, utilize fewer components, further reducing part size and loss.
This invention is directed to improved lumped-element RF components such as tunable filters and phase shifters. In particular, the invention involves blending a CPW nature in with lumped element circuits. The ground structure becomes very important when blending CPW in components with at least three inductors, more importantly, when four or more inductors are included in the structure, and is even more particularly important when there are cascaded lowpass/highpass filters and cascaded phase shifters. The operating frequency of these devices is generally in the range of 2-35 GHz, but broadly is in the range of 0.5-50 GHz, or greater.
The core of lumped-element tunable dielectric devices is a tunable capacitor, in which a tunable dielectric material, more commonly when BST is the variable dielectric. Variable dielectric capacitors can be fabricated using parallel plate or planar configurations. Parallel plate thin film structures use low DC voltages for tuning (e.g., <20 V), but the fabrications are sophisticated due to the patterning and etching of the Pt bottom electrode normally used with BST. Furthermore, the growth of a highly crystalline and defect-free BST thin films requires a chemically compatible bottom electrode that is electrically conductive at microwave frequencies. Platinum has been the primary bottom electrode for the parallel-plate BST capacitors, but its poor conductivity leads to high device losses. On the other hand, planar configurations require fewer lithography steps and allow for thicker metal to be deposited for lower metal losses. Most importantly, epitaxial films can be deposited onto single crystal substrates ensuring lower dielectric losses and higher dielectric constants. Deposition techniques, such as pulsed laser deposition (PLD) and metalorganic chemical vapor deposition (MOCVD), have been employed to grow epitaxial films on expensive substrates such as MgO and LaAlO3. These processes, however, are expensive, require costly starting materials, and have low throughput. Sputtered films often require post-deposition annealing to improve crystalline quality.
The recently developed open atmosphere, low cost combustion chemical vapor deposition (CCVD) technology offers an attractive alternative to grow epitaxial BST thin films on inexpensive sapphire substrates with good yield and high throughput potential. CCVD deposition of BST thin films is described, for example, in U.S. Pat. No. 6,986,955. Another tunable dielectric material useful as the dielectric for tunable capacitors is bismuth zinc niobate (BZN). Bismuth zinc niobate belongs to a category of oxide pyrochlore with a general formula of A2B2O6O′ (O′=seventh oxygen), which is not ferroelectric. BZN exhibits moderate permittivity (170-200), very low loss (tan δ˜5×10−4) and large tunability (55%) at low frequencies (˜1 MHz). Furthermore, its temperature coefficient of capacitance can be adjusted between −400 and 200 ppm/° C. through composition variations and process control.
With the pervasive growth of electronic systems in the military, commercial, and public safety marketplaces, there is a growing need for low cost devices with reduced size, weight, and power (SWAP), yet added functionality and superior performance. Making spiral inductors greatly reduces size of lumped elements components, but symmetry is reduced with spiral versus straight inductors. The ground structures or the present invention allow the proper performances with non-symmetrical designs even when a CPW theme is used for the structure of the main signal.
One important application of the invention is formation of tunable filters. Microwave filters are widely used in radar, communications, test equipment, and electronic warfare (EW) systems. In the case of high frequency receivers, filters must be used to remove signals in unwanted frequency bands to prevent overload of the receiver itself and to prevent undesirable interference from signals outside the band of operation. On the transmit side, signal purity must be maintained in order to minimize interference to other users, to conform with government regulations for radio emissions, and, in the case of military applications, to minimize detection of the radio source by hostile forces.
Tunable filters are used in all major areas of microwave engineering. Most tunable filters described in the literature fall into three basic types: mechanically tunable, magnetically tunable, and electronically tunable filters. Compared to mechanically and magnetically tunable filters, electronically tunable filters can be tuned very fast over a wide (an octave) tuning range, and they offer compact size. Most of the electronically tunable filters use varactor diodes and at higher frequencies these are based on GaAs. The varactor diode capacitance varies with reverse voltage; when a varactor is in series with a resonator circuit or element, this capacitance change alters the resonant frequency. However, these varactor-tuned filters have low power handling capability and consume power for operation.
In recent years, tunable dielectric and radio frequency microelectromechanical systems (RF MEMS) technologies have emerged as a promising candidate for tunable microwave device applications. They both show high power handling capability, negligible power consumption, and high isolation.
Ferroelectric BST-based tunable filters have been demonstrated by a number of groups. RF MEMS switch based tunable filters have shown much lower loss, but they tend to exhibit discrete and slow tuning, and can have reliability problems.
For modern multiband and multifrequency communication systems, reconfigurability in frequency and bandwidth is desired. The most widely used tunable techniques involve variable resistance elements to produce continuous tuning and coupling variation by means of varactor diodes. Switchable bandwidth has been realized by means of pin diodes and inter-digital coupled resonators. Herein is described cascading of a tunable lowpass filter (LPF) and a tunable highpass filter (HPF) to achieve a bandpass filter (BPF) or a bandstop filter (BSF). By tuning the LPF and HPF separately, not only is the center frequency of the BPF (or BSF) tuned, but also its bandwidth is varied when either LPF or HPF is tuned.
LPF is a filter that passes lower frequencies, and rejects higher frequencies. A series inductor or shunt capacitor or combination of the two is a simple low-pass filter. An HPF passes higher frequencies and rejects lower ones. A series capacitor or shunt inductor or combination of the two is a simple high-pass filter. Several popular filter functions are Chebyshev (equal-ripple amplitude), Bessel-Thomson (maximally flat group delay), Butterworth (maximally flat amplitude), and Gaussian.
Another important application of the invention is phase shifters formed of multiple thin films. Phase shifters are an essential component in electronically scanned phased-array antennas for communication and radar applications, and typically represent a significant amount of the cost, size and weight of producing military tactical antenna array. Jamming and interferers of mobile communication devices can be eliminated via phase shifters while still receiving the desired signal even with the interferer being at the same frequency.
Compared to mechanically and magnetically tuned phase shifters, electronically tunable phase shifters are compact, have fast response, and consume less power. Semiconductor-based phase shifters use pin diodes, GaAs varactors, or metal-oxide-semiconductor field-effect transistors (MOSFET) as the switching or tuning element. Even though these devices are inexpensive, they have higher loss at K-band or higher frequency bands and low power handling. Microelectromechanical switches (MEMS) use advanced integrated circuit (IC) processing techniques, which offer potential integration with GaAs monolithic microwave integrated circuits (MMIC) or MOSFET technologies. They provide low insertion loss, high isolation, negligible power consumption, and low intermodulation distortion (IMD); however, they require high driving voltage (e.g. 40 V or higher), have low switching speed (>10 μs), and can suffer from discontinuity, reliability, packaging, and variable g-force problems. The present invention applies to all variable capacitor based structures that can be varied by any afore mentioned methods for making tunable capacitors or inductors, but tunable dielectrics are the preferred embodiment.
Tunable dielectric materials exhibit inherently tunable capacitance through only a single DC bias voltage, and these tunable dielectric devices have continuity (analog), fast tuning speed, low loss, durability, low power consumption and high power handling. All these unique characteristics have attracted attention for microwave applications such as tunable filters, phase shifters, matching networks, and oscillators. Nevertheless, the progress made on tunable dielectric devices has been elusive. The problem is multi-faceted: (i) the loss was still high compared to its counterparts, attributed to the poor crystalline quality, (ii) a process of making high-quality, large-area BST at reasonable cost did not exist, and/or (iii) the required DC bias voltage was too high (˜100 V) or IMD was an issue.
Epitaxial BST films on commercially practical sapphire wafers, described in above-referenced U.S. Pat. No. 6,986,955, has demonstrated low loss and high tunability. Low-voltage (<20 V) capacitor structures that displayed improved IMD performance are described in U.S. Pat. Nos. 6,986,955, 6,970,500, and 7,031,136.
Several design options for variable capacitor phase shifters have been proposed. Reflection-type phase shifters consist of a 3-dB coupler and reflective loads. While wide bandwidths may be achieved with the reflection type topology, the coupler contributes directly to the insertion loss of the phase shifter, and requires a large portion of the die area. Loaded-line phase shifters are controlled by varying the capacitive loading on a coplanar waveguide transmission line. The phase shifters using this topology tends to become long at low frequencies (<10 GHz). Phase shifters have been designed using an all-pass network (APN) topology, which exhibit small size and low loss.